Add "line interval selection" based file diff feature
It may happen the need to compare big code files, when the actual necessity is just to compare a few functions in these files (clearly, the files could differ -even a lot- in other parts, and we are not interested in these ones, this may happen when doing refactoring or porting from legacy code). Without doing copy/paste into different files every time, a line interval selector in the diff view would be useful in order to allow comparing only relevant intervals of the two files (e.g. choosing lines l1 to l2 of file 1 to be compared against lines l3 to l4 of file 2).